The present invention relates generally to a light emitting semiconductor device and a method for fabricating the same. More specifically, the invention relates to a light emitting compound semiconductor device, which has an electrode having good adhesion and a low contact resistance, and a method for fabricating the same.
Semiconductors with the composition In.sub.x Al.sub.y Ga.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1), which are Group III-V nitride compound semiconductors, are direct gap semiconductors, and have a band gap varying in the range of from 1.89 to 6.2 eV by controlling x and y of the composition, so that these semiconductors are promising materials for LEDs (light emitting diodes) and semiconductor laser diodes. In particular, if the semiconductors can emit light at a high luminance in a wavelength region of blue, it is possible to or increase the storage capacity of various optical memory disks, and it is possible to realize a full color display. Therefore, blue light emitting semiconductor devices with the composition In.sub.x Al.sub.y Ga.sub.1-x-y N have been rapidly developed to stabilize their characteristics and to improve their reliability. Throughout the specification, it is assumed that the compositions "In.sub.x Al.sub.y Ga.sub.1-x-y N" include all the compositions wherein the composition ratios x and y vary in the range of from 0 to 1. For example, it is assumed that the compositions "In.sub.x Al.sub.y Ga.sub.1-x-y N " also include GaN (x=0, y=0).
References disclosing the structures of conventional blue light emitting device of nitride semiconductors include: Jpn. J. Appl. Phys., 28(1989)p. L2112; Jpn. J. Appl. Phys., 32(1993)p. L8; and Japanese Patent Laid-Open No. 5-291621.
In light emitting devices, an electrode section for supplying driving current has a very important role for the characteristics of the light emitting devices. In nitride compound semiconductors, it is particularly important to select the structure and material of an electrode for the nitride compound semiconductors, since it is generally difficult to obtain a good ohmic contact.
For example, in the aforementioned Jpn. J. Appl. Phys., 28(1989)p. L2112, aluminum (Al) is used as an electrode material for an n-type In.sub.x Al.sub.y Ga.sub.1-x-y N layer. In the aforementioned Jpn. J. Appl. Phys., 32(1993)p. L8, gold (Au) is used as the electrode material. In the aforementioned Japanese Patent Laid-Open No. 5-291621, any one of chromium (Cr), titanium (Ti) and indium (In) is used as the electrode material.
However, if aluminum, chromium, titanium or indium is used as the electrode material for the n-type In.sub.x Al.sub.y Ga.sub.1-x-y N layer, there is a problem in that the contact resistance is relatively high in all cases. That is, if an electrode of aluminum is used to prepare a LED to evaluate its current/voltage characteristics, the differential resistance thereof is as high as hundreds .OMEGA. at a current of 20 mA. The current density of a semiconductor laser diode must be higher than that of a LED, so that it is required to further decrease the contact area of the electrode. Therefore, in the case of the semiconductor laser, the differential resistance in the current/voltage characteristics is further increased. As a result, there is a problem in that the operating voltage of the laser rises to increase its threshold current due to heat generation and to saturate an optical output power. This problem is not caused only in the case of the electrode of aluminum, but it is also caused in the case of an electrode of chromium, titanium or indium. There is also a problem in that the crystallinity of a semiconductor layer deteriorates if the carrier concentration in the semiconductor layer is increased to decrease the contact resistance. For example, in order to increase the carrier concentration in an n-type In.sub.x Al.sub.y Ga.sub.1-x-y N layer, if impurities are doped higher than or equal to 1.times.10.sup.19 cm.sup.-3, the crystallinity of the layer deteriorates and the surface morphology thereof also deteriorates. In a typical blue nitride semiconductor device, an n-type semiconductor layer is used as an underlayer, and p-type semiconductor layers are sequentially epitaxial-grown thereon. Therefore, if the crystallinity of the n-type semiconductor layer serving as the underlayer deteriorates, the surface morphology thereof deteriorates, and the crystallinity of the semiconductor layers, such as an active layer, formed thereon also deteriorates, so that there is a serious problem in that it is not possible to obtain good emission characteristic.
On the other hand, there is a problem in that the above described conventional electrode structure has a relatively high contact resistance. In particular, it is not easy to provide a good ohmic contact for the p-type In.sub.x Al.sub.y Ga.sub.1-x-y N layer,. Therefore, the same problems as the above problems are caused by the high contact resistance in a n-side electrode as well as in a p-side electrode.
Moreover, since the conventional electrode does not have a sufficient bond strength to the In.sub.x Al.sub.y Ga.sub.1-x-y N layer, there are problems in that the device resistance is easy to increase and the electrode is easy to be peeled off. Therefore, there is a problem in that the initial characteristics of the light emitting device is not only deteriorated, but the reliability thereof is also deteriorated. Moreover, if the bond strength of the electrode is not sufficient, it is difficult to achieve a so-called flip-chip mounting. Therefore, there is a problem in that it is not possible to achieve the improvement of electrical and optical characteristics and the reduction of packaging dimension, which are able to obtained by the flip-chip mounting.